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Summary:
Instead of culling faces behind the camera, partially clip them if they intersect with the image plane.

This diff implements the utils functions for clipping.

There are 4 cases for the mesh faces which are all handled:

```
Case 1: the triangle is completely in front of the clipping plane (it is left
        unchanged)
Case 2: the triangle is completely behind the clipping plane (it is culled)
Case 3: the triangle has exactly two vertices behind the clipping plane (it is
        clipped into a smaller triangle)
Case 4: the triangle has exactly one vertex behind the clipping plane (it is clipped
        into a smaller quadrilateral and divided into two triangular faces)
```

Reviewed By: jcjohnson

Differential Revision: D23108673

fbshipit-source-id: 550a8b6a982d06065dff10aba10d47e8b144ae52